Paving stones of the type to which the present invention relates are manufactured slab or paving elements usually molded of ceramic material, most commonly concrete, into predetermined shapes which, when arranged in a pattern, form a covering for the ground or other surface area which is generally intended to bear pedestrian or vehicular traffic.
Bricks, cut stones and slab elements of various types have long been used to cover roads and walkways to form a pavement or ground cover arrangement. In forming the ground cover pattern, the elements are preferably laid adjacent each other in an array to fully cover the area being paved. The most common shape of element used historically is the rectangular brick like shape which can easily be arranged to fully cover the ground without resort to combinations of stones of different sizes or shapes to do so. Such elements are laid with or without grout or mortar joints which rigidly join one element with another.
A type of ground cover finding increasing use is that formed of the paving stones laid without mortar or grout, usually with joints filled with particulate material such as sand. The advantages which such ground covers present are an ability to tolerate movement and deformation without exhibiting the cracking and breaking which may result with ground covers in which rigid grout or mortar joints are employed.
One disadvantage found with the simple rectangular elements such as bricks and rectangular stones is that, when used with sand or other loose fill joint material, surface water flowing on the pavement area formed of such a ground cover has a tendency to wash the joint material from between the elements. Another disadvantage is that the elements have a tendency to tilt or yield under locally heavy loads.
One solution to both the problem of the washing of joint material from between the elements and to the problem of movement under load has been the introduction of mortarless or groutless paving stones of the interlocking type. Such interlocking paving stones are for example those disclosed in the U.S. Pat. Nos. of Hair 4,544,305 and Barth 4,128,357. An objective in the design of interlocking paving stones, as seen in the Hair and Barth patents, is the creation of shapes which will interlock in such a way as to fully cover the area being paved with a minimum of different stone shapes. It is highly desirable that stones of a single size and shape be capable of forming an interlocking pattern which fully covers the ground. Such a characteristic reduces the number of costly molds and the need for distributors and installers to maintain inventories of different stones. It has also been an objective, difficult in many cases to achieve, to shape the stone in a way that it will not only interlock satisfactorily and form a pattern which fully covers the area being paved, but which will do so with shapes which present boundaries which contribute to a particular aesthetic pattern. By the very nature of the stones, the boundaries which define their shapes make the primary contribution to the overall appearance of the patterns. Unfortunately, not all aesthetically desirable shapes are easily made to interlock effectively. The desire to provide certain shapes in paving stones makes it difficult to design stones which interlock effectively. Thus, the desire to form patterns which yield certain aesthetic effects imposes a constraint on the stone characteristics which preclude the utilitarian properties for which the interlocking stones are desired.
Paving stones of the prior art have generally been of polygonal shapes so that the straight sides will more easily abut those of an adjacent, preferably identical, stone when arranged to from a ground cover pattern. Both the Hair and Barth stones described in the patents identified above are in this category. Shapes with curves have not been successfully developed which form aesthetically acceptable patterns while interlocking in the formation of a pattern to cover the ground. This has been particularly the case with shapes which employ circles and are bounded by circular or cylindrical sides. The Hair stone described above has, as one of its advantages, the ability to provide radial patterns which produce, on a large scale at least, curved and circular appearances. Nonetheless, stones which themselves have prominent curved surfaces capable of influencing the overall patterns of the ground cover have been difficult to design so as to interlock effectively.
Accordingly, there has existed a need for an interlocking paving stone with curvilinear sides which will fully cover the ground with a minimum number of different stone shapes in the pattern.